The present invention relates generally to cooking apparatus, and more particularly provides tunnel oven apparatus for cooking food items, such as pizzas, in which a substantially improved impingement air delivery system is provided for rapidly, efficiently and very evenly cooking the food items from above and below the same as the food items are moved through the oven. The air delivery system may also be used to efficiently deliver cooled impingement air to the food items, if desired, to rapidly cool them as they traverse the oven's tunnel.
Tunnel ovens are well known in the food preparation industry and basically comprise an elongated horizontal housing through which a cooking chamber or "tunnel" longitudinally extends. The food items to be cooked are moved through the cooking tunnel on a conveyor apparatus which subjects the food items to a predetermined tunnel temperature for a predetermined amount of time. A now-conventional method of supplying cooking heat to the food items moving through the tunnel is the use of heated, high velocity impingement air which is directed against upper and lower surface portions of the moving food items. The high velocity cooking air is delivered in the form of columnar jets having a circular cross-section and a velocity on the order of 8000 feet per minute.
These high velocity columnar jets are formed by utilizing a single fan to supply heated air into the inner ends of duct plenums or "jet fingers" positioned above and below the food conveyor belt and extending generally transversely thereto. The vertically facing surfaces of these upper and lower duct plenums are defined by columnating plates having circularly cross-sectioned jet-forming orifices positioned thereon in a suitably spaced array. Air flow into the upper and lower duct plenums from the single supply fan forces the heated air vertically through the jet-forming orifices in the form of high velocity circular jet columns which strike the upper and lower surfaces of the food items in a substantially undiffused form. This columnar jet impingement scheme is also used to cool food items traversing the tunnel, the only substantial difference being that the air supply to the upper and lower duct plenums is cooled rather than heated.
Despite the widespread use of this impingement air delivery technique, this conventional method of heating or cooling food items conveyed through an elongated tunnel is subject to a variety of well known problems, limitations and disadvantages. For example, the columnated, high velocity, substantially undiffused air jets strike the food items at spaced apart surface locations thereon, in a pattern corresponding to the orifice pattern on the columnating plates, and tend to unevenly cook the food items despite the fact that such food items are being moved relative to the columnar jet pattern by the conveyor belt. More specifically, as the food items are moved relative to the high velocity jets, a "streaked" cooking pattern is unavoidably obtained on the upper and lower surfaces of the food items.
This, however, is not the only cooking unevenness problem. It is well known that it is quite difficult to balance the air volume flow through the individual orifices of each columnating plate. This volume imbalance is most prevalent along the length of the upper and lower supply duct plenum and results in the orifices adjacent the outer end of each plenum discharging significantly more impingement air than the orifices located near the inner end of such plenum. This results in a cooking temperature imbalance laterally across the conveyor belt such that items close to one edge of the belt are cooked more than items positioned adjacent the opposite edge thereof.
Attempts have been made to rectify this lateral air delivery imbalance by vertically tapering the facing supply duct plenums so that the outer ends thereof are vertically narrower than their inner ends which are connected to the supply fan. While this somewhat alleviates this particular air imbalance problem, it does not completely solve it. This is due to the fact that the flat columnating plates, with their circular jet-forming orifice members, are quite sensitive to the flow characteristics of air being supplied to the duct plenums which they partially define. More specifically, it has been found that it is rather difficult, even with the tapered duct design, to achieve uniform air volume discharge through the various orifices except within a rather limited air inlet velocity range.
Stated otherwise, the duct plenums utilizing these columnating plates must be "tuned" to a particular and rather narrow plenum inlet velocity range. An inlet velocity which is either too high or too low will cause a significant air discharge imbalance among the outlet orifices in a given supply duct plenum. The jet-forming orifices are also quite sensitive to the presence of air turbulence within the particular supply plenum. Thus, even if the inlet velocity to the plenum is within the predetermined "tuned" range, turbulence in the inlet air flow will also unbalance the air discharge rates among the orifices in the plenum.
These various air discharge imbalance problems result in unavoidable cooking imbalances laterally across the conveyor belt as well as imbalances between the upper and lower surfaces of a given food item. These imbalance problems, of course, are also present when food cooling air is being supplied through the upper and lower plenums.
It is often desirable to heat (or cool) the upper and lower surfaces of food items at different rates as they traverse the tunnel. This adjustment is typically achieved by varying the relative amounts of air supplied to the upper and lower duct plenums. For example, if it is desired to lower the cooking (or cooling) rate on the upper surface of the food items, the volume of air supplied to the upper duct plenum is reduced relative to the air supplied to the lower duct plenum. This is conventionally accomplished by providing plenum inlet dampers to selectively vary the inlet volume to each of the duct plenums. However, the use of such inlet dampers unavoidably alters the plenum inlet velocity and imparts varying degrees of air turbulence within the plenums. Thus, while the total air discharged from either of the duct plenums may indeed be varied, this air supply regulation scheme can and often does alter the critical velocity "tuning" of either or both of the supply duct plenums which, in turn, creates an air discharge imbalance along the length of either or both of the plenums.
It is also often necessary to vary the relative air supply volumes between the upper and lower duct plenum in this manner to compensate for food items having varying heights. Specifically, higher items (which are positioned closer to the upper supply duct plenum) must have, along their upper surfaces, a lower air impingement velocity and/or a lesser volume of heated or cooled air emanating from the upper supply plenum duct. However, regulation of the relative volumes discharged from the upper and lower duct plenums unavoidably creates the orifice supply imbalance problem previously mentioned.
In addition to these air flow control problems, conventional air impingement tunnel ovens utilizing this columnated jet supply scheme are also subject to problems arising from the very high velocity of the heating or cooling air jets. Specifically, because of the very high velocity thought necessary to achieve proper impingement air cooling or heating, the jets, after impinging upon and being deflected from the food items, are left with a residual velocity which is still quite high. This tends to stratify the air flow within the oven which results in the possibility of "starving" and cavitating the supply fan used to deliver heated or cooled air to the upper and lower supply duct problem can necessitate the rebalancing of the impingement air system to satisfy the fan operating characteristics. In turn, however, this can lead to the uneven heating or cooling characteristics previously mentioned.
Another problem heretofore associated with this columnated jet type of food heat treatment apparatus is that it is essential to keep the food items moving relative to the circularly cross-sectioned high velocity air jets to avoid even more unevenness in the food cooking or cooling process. Specifically, if the food items are stopped for any appreciable length of time within the tunnel, the columnar jets will simply cook (or cool) the upper and lower surfaces of the food items only or discrete, spaced locations thereon. This inability to heat or cool the food items in a stationary position within the tunnel, of course, greatly limits the operational flexibility of the oven since it may be desirable in many instances to provide a longer cooking time than that available with a given tunnel length and conveyor system speed.
A further limitation associated with conventional air impingement tunnel oven cooking apparatus is the inability to safely grill meat items therein, the cooking capability of such apparatus being essentially limited to baking operations in which no appreciable amounts of grease are generated. This is due to the fact that because of the necessarily high velocity of the columnar air jets, and the uneven cooking patterns resulting therefrom, it has been necessary to elevate the air jet temperature to above the combustion "flash point" of grease vapor to achieve an actual grilling of meat products traversing the oven tunnel. Because of this necessity, grease fires are easily started in conventional air impingement tunnel ovens and they are accordingly not normally utilized to grill meat products.
It is accordingly an object of the present invention to provide tunnel oven apparatus which has an improved impingement air delivery system that eliminates or minimizes above-mentioned and other problems and limitations associated with conventional columnar jet high velocity impingement air systems.